Porcelain ceramics have been used in dentistry for several hundred years. They are well known for their ability to mimic the appearance of a natural tooth. In the field of fixed, cemented dental prosthetic restorations, such as crown and bridges, porcelain has commonly been used as a single tooth replacement in the form of a porcelain jacket crown (PJC). A PJC consists entirely of porcelain and is cemented onto a prepared tooth. Due to the mechanical properties of the PJC, it has generally not been used for posterior teeth. In the case of a bridge type of restoration, the porcelain initially consisted of prefabricated and individually adjusted facial veneers fitted to the bridge structure in the wax up stage. After casting the alloy bridge structure, these facial veneers were cemented onto the alloy structure by the use of a zinc-phosphate cement that provided mechanical retention.
In the 1950's the porcelain-fused to-metal restoration was introduced. Generally, a particulate suspension of porcelain ceramic is applied onto a thin pre-oxidized crown and bridge alloy substructure having a compatible coefficient of thermal expansion. The porcelain ceramic is then fired in a furnace. Upon firing, the porcelain is vitrified onto the pre-oxidized surface of the alloy substructure and the materials are bonded together. The bonding mechanism has not yet been completely explained. The shear bond strength of porcelain bonded to an alloy substructure has been reported to be in the neighborhood of 13,000 PSI (Anusavice K., Screening Tests for Metal-Ceramic Systems, p. 373-387. Dental ceramics, Proceedings of the First International Symposium on Ceramics, Quintessence, Chicago, J. W. McLean, ed. 1983).
The porcelain ceramic is generally applied in several layers to achieve a desirable aesthetic effect. The first layer is usually an opaque porcelain ceramic containing a relative high amount of opacifying refractory ceramic material. This layer, commonly called opaque, is most often fired separately and obscures the underlying alloy surface. Thereafter, body and incisal porcelain may be applied by any of several methods, including by "flame spray" technique, or by a "paint-on" or "spray-on" technique, followed by firing.
By utilizing an underlying alloy substructure the porcelain is reinforced in such a way that it is also possible to apply these ceramo-metal restorations in the posterior area of the mouth, and fabricate entire bridges in alloy-reinforced porcelain. Today, the ceramo-metal restoration is perhaps the most common fixed dental prosthetic restoration used. However, despite the many advantages of using porcelain as a dental restorative material, it has some significant drawbacks. Due to its microstructure and hardness, it has a tendency to be highly abrasive on natural dentition, being in opposing contact during mastication. It is brittle, and difficult to successfully repair when fractured in the mouth. Ceramo-metal restorations are also difficult and time consuming to fabricate in the dental laboratory.
Developments and improvements in polymerizable resins and composite resins have enabled these materials to become increasingly attractive as alternative materials for porcelain. However, the mechanical properties of these materials do not allow their use for sole formation of entire permanent fixed restorations. Instead, they are usually either applied as a facial veneering material onto a cast crown and bridge replacement alloy structure, or they are used in combination with a thin, underlying, reinforcing alloy substructure.
Generally, the prior art applications result in structures that are secured either by mechanical or chemical bonding, and they provide generally for a poor resin or composite resin to alloy interface. Each of these known methods may have drawbacks in use or application, or result in a bond which has either inadequate initial strength or a tendency to deteriorate after time. To improve bond strength with these methods, it is conceivable that some of them may be combined.
Mechanical retention may be facilitated by the provision of loops, wires, mesh, beads, or crystals. Such retentions are reviewed in two periodicals, including Shue, Nichols, Townsend, J. Prosth. Dent., Vol. 58, No. 3, September, 1987, pages 297-305; and Naka, C. K., Gen. Dent., July-Aug. 1987, pages 307-310. These mechanical devices are placed in the form of wax or plastic patterns on the wax-up, and cast to form the alloy structure or substructure. The major inherent shortcoming of this and similar methods is that they provide for only mechanical retention between the restoration components. Due to differences in mechanical properties, and to the fact that resins will contract upon polymerization, microscopic gaps may occur at their interfaces. Oral fluids seeping into this gap may cause severe interfacial discoloration or, in extreme cases, a complete delamination of the resin material from the alloy structure or substructure. These types of retentions may compromise the space for the resin or composite resin materials, since they themselves require extensive space.
Mechanical retention without the need for loops or the like is possible through acid etching of the crown and bridge substructure. The alloy structure or substructure is washed with or immersed in acid, and micropores form in the alloy through etching or electrolytical etching. When semi-liquid resin or composite resin material is placed on this substructure, a portion of it enters the micropores. Upon hardening, the material is locked in the micropores and a mechanical bond between the resin or composite resin and the alloy is formed. This method is generally described in Tanaka, Atsuta, Uchiyama, and Kawashima, J. Prosth. Dent., Vol. 42, No. 3, September, 1979, pages 282-291. There are three main disadvantages to this method. First, only predominant base alloys can be utilized, as noble alloys are not etchable in this manner. Second, it is difficult to restrict the acid and the resulting micropore formation to those areas where it is desirable. Third, the bond is purely mechanical.
A chemical bonding method is believed to have been introduced by Musil and Tiller, of Kulzer & Co. GmbH, Wehrheim, Federal Republic of Germany, in 1984. One procedure has been described in Musil and Tiller, Dent. Labor., Vol 32, pages 1155-1161, 1984. This method involves silicoating, i.e., the flame-spray deposition of organic silicone molecules to an alloy structure or substructure. The silicone layer is then treated with a silane coupling agent, which provides the means for formation of the chemical bond. Finally, a polymerizable resin or composite resin is applied to the structure or substructure. This method has questionable long-term durability, as it appears that water attacks the interfacial bond. Particularly, it has been reported that specimens stored for 90 days in 37.degree. C. distilled water lost 30% of their original bond strength. Hero, Ruyter, Waarli, Hultquist, J. Dent. Res. 66(8): pages 1380-1385, August, 1987.
Other bonding methods, combining chemical and mechanical bonding, particularly utilizing a composite resin luting cement, are also known. For example, intra-oral porcelain-fused-to-metal restorations have been repaired by etching the surface of the porcelain, and then cementing a prefabricated replacement porcelain laminate facing with a composite resin luting cement. A silane coupling agent is used in connection with the composite resin luting cement, apparently to improve retention by providing a chemical bond. Nixon, Dent. Today. December, 1986, pages 27 and 31. Nixon teaches that the surface must be contoured with a diamond instrument, that dilute hydrofluoric acid may be used for etching, that at least 75% of the porcelain-fused-to-metal unit should remain after contouring, and that the remaining porcelain-metal unit should be predominantly comprised of porcelain. Methods of porcelain repair using only silane, without etching the porcelain to create micropores in the porcelain surface, were reviewed by Ferrando. In this case a composite restorative material was used instead of a porcelain laminate facing. Calamia and Simonson (IADR, abstract 1095, J. Dent. Res., vol. 63, pages 172-362, 1984) and Stangel, Nathanson, and Hu (J. Dent. Res. volume 66, number 9, pages 1460-1465, September, 1987) have also reported on the bond strength of a composite resin luting cement bonded to an etched and silane-treated surface.
A natural tooth may also be restored or altered by first etching the tooth, and then securing a prefabricated porcelain laminate facing to that etched tooth with a composite resin luting cement. Horn, Dent. Clin. North Am., volume 27, pages 671-684, 1983; and Calamia, N.Y. J. Dent., volume 53, pages 255-259, 1983.
To the knowledge of the inventor, none of the prior art teaches a method comprising the application of a thin ceramic layer to act as a bonding medium between a polymerizable resin or composite resin material and a reinforcing alloy substructure, or to a dental crown and bridge alloy structure so as to retain a composite resin veneering material to that structure.